Rubber reinforcing cords have been made of organic fibers, such as rayon, polyamide, polyester, and, as the lastest addition, aramide, or inorganic fibers such as glass fiber, steel fiber, etc. For particular use in tires, cords for rubber reinforcement are desirably made of fibrous materials having high strength, high modulus, and lightweight, from the standpoint of controllability and running stability of tires when driving, comfortability to ride on, durability of tires, and fuel cost.
Carbon fibers have a lower density, higher modulus of elasticity, and higher strength as compared with the above-recited glass fiber and steel fiber and are, therefore, highly promising as an excellent reinforcing material for rubber.
However, carbon fibers have a disadvantage of poor adhesion to rubber. In order to overcome this disadvantage, various improved processes for producing reinforcing cords from carbon fibers have hitherto been proposed, such as a process comprising twisting elastomer-impregnated carbon fibers as disclosed in U.S. Pat. No. 3,648,452; a process comprising treating carbon fibers with an epoxy compound and then with an adhesive, such as a mixture of a resorcinol-formaldehyde condensate and a rubber latex (hereinafter referred to as RFL) as disclosed in Japanese Patent Application (OPI) No. 102678/75 (the term "OPI" as used herein means "unexamined published application"); a process comprising treating carbon fibers with a first treating bath containing a polyisocyanate and then with a second treating bath containing RFL as described in Japanese Patent Application (OPI) No. 102679/75; and the like. Nevertheless, none of these attempts have completely succeeded in attaining fully satisfactory adhesion to rubber. In particular, carbon fiber cords are inferior in resistance to repeated fatigue, such as flexual, compression, and the like, due to the high modulus of elasticity of carbon fibers.
According to the inventors' study, it was found that the insufficient performances of the above-described conventional carbon fiber cords arise from insufficient adhesion or bonding between carbon fibers and rubber, the elastomer, or the polyisocyanate. Moreover, although bond of adhesion between epoxy resins and carbon fibers is relatively strong, the epoxy resin-treated carbon fibers do not have softness any more, and, when applied to rubber reinforcement, exhibit rather deteriorated resistance to flexing fatigue.
Since the carbon fibers as treated with the epoxy resin have poor adhesion to rubber, an additional treatment with RFL is required. According to this technique, the uncured epoxy resin is reacted with RFL to increase adhesion strength to rubber, but there is noted a tendency that the resulting cord has reduced softness and reduced fatigue performance.